The group of pre-graphitic compounds includes carbonaceous materials that are generally prepared at low temperatures (eg: less than about 2000.degree. C.) from various organic sources and that tend to graphitize when annealed at higher temperatures. There are however both hard and soft pre-graphitic carbon compounds, the former being difficult to graphitize substantially even at temperatures of order of 3000.degree. C., and the latter, on the other hand, being virtually completely graphitized around 3000.degree. C.
The aforementioned set of compounds has been of great interest for use as anode materials in lithium-ion or rocking chair type batteries. These batteries represent the state of the art in small rechargeable power sources for consumer electronics applications. These batteries have the greatest energy density (Wh/L) of conventional rechargeable systems (ie. NiCd, NiMH, or lead acid batteries). Additionally, lithium ion batteries operate around 31/2 volts which is often sufficiently high such that a single cell can suffice for many electronics applications.
Lithium ion batteries use two different insertion compounds for the active cathode and anode materials. Insertion compounds are those that act as a host solid for the reversible insertion of guest atoms (in this case, lithium atoms). The structure of the insertion compound host is not significantly altered by the insertion. In a lithium ion battery, lithium is extracted from the anode material while lithium is concurrently inserted into the cathode on discharge of the battery. The reverse processes occur on recharge of the battery. Lithium atoms travel or "rock" from one electrode to the other as ions dissolved in a non-aqueous electrolyte with the associated electrons travelling in the circuit external to the battery.
The two electrode materials for lithium ion batteries are chosen such that the chemical potential of the inserted lithium within each material differs by about 3 to 4 electron volts thus leading to a 3 to 4 volt battery. It is also important to select insertion compounds that reversibly insert lithium over a wide stoichiometry range thus leading to a high capacity battery.
A 3.6 V lithium ion battery based on a LiCoO.sub.2 /pre-graphitic carbon electrochemistry is commercially available (produced by Sony Energy Tec.) wherein the carbonaceous anode can reversibly insert about 0.65 Li per six carbon atoms. (The pre-graphitic carbon employed is a disordered form of carbon which appears to be similar to coke.) However, the reversible capacity of lithium ion battery anodes can be increased by using a variety of alternatives mentioned in the literature. For example, the crystal structure of the carbonaceous material affects its ability to reversibly insert lithium (as described in J. R. Dahn et al., "Lithium Batteries, New Materials and New Perspectives", edited by G. Pistoia, Elsevier North-Holland, p1-47, (1993)). Graphite for instance can reversibly incorporate one lithium per six carbon atoms which corresponds electrochemically to 372 mAh/g. This electrochemical capacity per unit weight of material is denoted as the specific capacity for that material. Graphitized carbons and/or graphite itself can be employed under certain conditions (as for example in the presentation by Matsushita, 6th International Lithium Battery Conference, Muenster, Germany, May 13, 1992, or in U.S. Pat. No. 5,130,211).
Other alternatives for increasing the specific capacity of carbonaceous anode materials have included the addition of other elements to the carbonaceous compound. For example, Canadian Patent Application Serial No. 2,098,248, Jeffrey R. Dahn et al., `Electron Acceptor Substituted Carbons for Use as Anodes in Rechargeable Lithium Batteries`, filed Jun. 11, 1993, discloses a means for enhancing anode capacity by substituting electron acceptors (such as boron, aluminum, and the like) for carbon atoms in the structure of the carbonaceous compound. Therein, reversible specific capacities as high as 440 mAh/g were obtained with boron substituted carbons. Canadian Patent Application Serial No. 2,122,770, Alfred M. Wilson et al., `Carbonaceous Compounds and Use as Anodes in Rechargeable Batteries`, filed May 3, 1994, discloses pre-graphitic carbonaceous insertion compounds comprising nanodispersed silicon atoms wherein specific capacities of 550 mAh/g were obtained. Similarly, specific capacities of about 600 mAh/g could be obtained by pyrolyzing siloxane precursors to make pre-graphitic carbonaceous compounds containing silicon as disclosed in Canadian Patent Application Serial No. 2,127,621, Alfred M. Wilson et al., `Carbonaceous Insertion Compounds and Use as Anodes in Rechargeable Batteries`, filed Jul. 8, 1994.
Recently, practitioners in the art have prepared carbonaceous materials with very high reversible capacity by pyrolysis of suitable starting materials. At the Seventh International Meeting on Lithium Batteries, Extended Abstracts Page 212, Boston, Mass. (1994), A. Mabuchi et al. have demonstrated that pyrolyzed coal tar pitch can have reversible specific capacities as high as 750 mAh/g at pyrolysis temperatures about 700.degree. C. K. Sato et al. in Science 264, 556, (1994) disclosed a similar carbonaceous material prepared by heating polyparaphenylene at 700.degree. C. which has a reversible capacity of 680 mAh/g. S. Yata et al., Synthetic Metals 62, 153 (1994) also disclose a similar material made in a similar way. These values are much greater than that of pure graphite. The aforementioned materials can have a very large irreversible capacity as evidenced by first discharge capacities that can exceed 1000 mAh/g. Additionally, the voltage versus lithium of all the aforementioned materials has a significant hysteresis (ie. about 1 volt) between discharge and charge (or between insertion and extraction of lithium). In a lithium ion battery using such a material as an anode, this would result in a similar significant hysteresis in battery voltage between discharge and charge with a resulting undesirable energy inefficiency.
It is not understood why the aforementioned carbonaceous materials have very high specific capacity. (However, J. Dahn et al., Electrochimica Acta, Vol. 3, No.9, p. 1179-1191, 1993 speculated on the possibility of certain unorganized carbons exceeding the capacity of graphite via lithium adsorption on single graphite layers contained within. Also, in the aforementioned reference by K. Sato et al., Li dimer formation was proposed as an explanation for the very high specific capacity of their carbonaceous material.) All these materials were prepared at temperatures of about 700.degree. C. and are crystalline enough to exhibit x-ray patterns from which the parameters d.sub.002, L.sub.c, a, and L.sub.a can be determined. (The definition and determination of these parameters can be found in K. Kinoshita, "Carbon--Electrochemical and Physicochemical Properties", John Wiley & Sons 1988.) Also, all have substantial amounts of incorporated hydrogen as evidenced by H/C atomic ratios that are greater than 0.1, and often near 0.2. Finally, it appears that pyrolyzing at higher temperature degrades the specific capacity substantially with a concurrent reduction in the hydrogen content. (In the aforementioned reference by Mabuchi et al., pyrolyzing the pitch above about 8000.degree. C. results in a specific capacity decrease to under 450 mAh/g with a large reduction in H/C. Similar results were found in the aforementioned reference by Yata et al.)
The prior art also discloses carbonaceous compounds with specific capacities higher than that of pure graphite made from precursors that form hard carbons on pyrolysis. However, the very high specific capacities of the aforementioned materials pyrolyzed at about 700.degree. C. were apparently not attained. A. Omaru et al, Paper #25, Extended Abstracts of Battery Division, p34, Meeting of the Electrochemical Society, Toronto, Canada (1992), disclose the preparation of a hard carbonaceous compound containing phosphorus with a specific capacity of about 450 mAh/g by pyrolyzing polyfurfuryl alcohol. The polyfurfuryl alcohol in turn had been prepared from the monomer polymerized in the presence of phosphoric acid. In Japanese Patent Application Laid Open number 06-132031, Mitsubishi Gas Chemical disclose a hard carbonaceous compound comprising 2.4% sulfur with a specific capacity of about 500 mAh/g. These hard carbonaceous compounds have additional elements incorporated therein and have all been pyrolyzed at sufficient temperature such that they contain little hydrogen (ie. the H/C atomic ratio is substantially less than 0.1). These hard carbonaceous compounds neither exhibited the very high specific capacities nor the same serious hysteresis in voltage of the aforementioned materials pyrolyzed at about 700.degree. C.
Additionally, other high capacity carbonaceous materials have recently been prepared which show high capacity for lithium and little or no voltage hysteresis. In Paper 2B05 at the 35th Battery Symposium in Nagoya, Japan, Nov. 14-16, 1994, Y. Takahashi et al. describe materials with reversible capacities of about 480 mAh/g, but do not give the details of their preparation. In paper 2B09 at the same Symposium, N. Sonobe et al. describe hard carbon materials made from petroleum pitch with reversible capacities near 500 mAh/g. The synthesis procedure therein was not given.
Japanese patent application laid open number 06-089721 discusses the high capacity advantages of hard disordered carbons in terms of the parameters P.sub.s (the fraction of stacked carbon), n.sub.ave (the number of graphene sheets per stack), and SI (the stacking index). Therein, SI is defined by the height of the {002} peak relative to the local background. Therein, carbonaceous compounds having values of SI below 0.76 were claimed and the examples provided had a minimum SI of 0.67. Reversible capacities for lithium up to 460 mAh/g were obtained. However, voltage curves (and hence hysteresis characteristics) and irreversible capacities were not reported. Additionally, discussion and data regarding hydrogen contents after pyrolysis and surface area accessible to non-aqueous electrolyte were not provided.